An integrated computational tool for precipitation simulation

Computer aided materials design is of increasing interest because the conventional approach solely relying on experimentation is no longer viable within the constraint of available resources. Modeling of microstructure and mechanical properties during precipitation plays a critical role in understanding the behavior of materials and thus accelerating the development of materials. Nevertheless, an integrated computational tool coupling reliable thermodynamic calculation, kinetic simulation, and property prediction of multi-component systems for industrial applications is rarely available. In this regard, we are developing a software package, PanPrecipitation, under the framework of integrated computational materials engineering to simulate precipitation kinetics. It is seamlessly integrated with the thermodynamic calculation engine, PanEngine, to obtain accurate thermodynamic properties and atomic mobility data necessary for precipitation simulation.     

Orientation image-based micromechanical modelling of subgrain texture evolution in polycrystalline copper

An efficient full-field formulation based on fast Fourier transforms (FFTs) for the prediction of the viscoplastic deformation of polycrystals is applied to the study of the subgrain texture and microstructure evolution in polycrystalline Cu deformed under tension. Direct input from orientation imaging microscopy (OIM) images is used in the construction of the initial unit cell. Average orientations and misorientations predicted after 11% tensile strain are directly compared with OIM measurements, showing good agreement. The differences between misorientations of surface grains compared with bulk grains are estimated, and the orientation dependence of intragranular misorientations is studied. Measurements and simulations agree in that grains with initial orientation near 〈1 1 0〉 tend to develop higher misorientations. This behavior can be explained in terms of attraction towards the two stable orientations and grain interaction. Only models that account explicitly for interaction between individual grains, like the FFT-based formulation, are able to capture these effects.     

Work-hardening model for polycrystalline metals under strain reversal at large strains

The mechanical behaviours under reversed strain of low carbon steels and aluminium alloys are reviewed and modelled with a simple approach based on the evolutionary laws of two dislocation densities related respectively to the forward and the backward straining. In essence, it is the competition between the annihilation of the dislocations that were created during the prestrain and the storage of newly created dislocations that lead to the observed stagnation of the hardening rate. Textural effects as well as back stresses are shown to extend or to reduce the stress–strain plateau but are not responsible for it.     

草酸镍前驱体纤维微结构诱导制备多晶态纳米镍纤维的演化机理

在离子液相沉淀体系中,通过受限自组装生长可控制备草酸镍纳米纤维。在氩气环境中,在纤维微结构诱导下,通过热分解草酸镍纤维制备出镍纳米纤维。通过X射线衍射仪、扫描电镜和透射电镜等技术检测所得样品的性质。结果表明,氨的存在使草酸镍的生长习性从自由自组装模式转变为了受限自组装模式,制备的草酸镍纤维具有单斜晶体结构,直径100-200nm,长度为1-5μm。 热分解制备的镍纤维表面粗糙,具有面心立方晶体结构,直径100-200nm,长度为1-5μm。在草酸镍纤维形成的细而长空间的诱导作用下,镍原子经过成核、生长和聚合过程,从而形成纤维状纳米镍。     

A polycrystalline mechanical model for bulk nanocrystalline materials using the energy approach

To systematically understand the grain size, strain rate and defect development dependent mechanical behavior of bulk nanocrystalline materials, a new constitutive model is proposed to describe the deformation mechanism, microstructure evolution and mechanical response of bulk nanocrystalline materials using the energy approach. In this model, the grain interior and grain boundary were not taken as two independent phases with different volume fractions, but as an integral object sustained dislocation and accommodated grain boundary sliding mechanisms. Meanwhile, defect creation and evolution and their effects on the overall stress–strain relation as well as the failure process of bulk nanocrystalline materials were considered in the model. For experimental verification, we have prepared nanocrystalline Ni powder by the DC arc plasma evaporation method. Bulk nanocrystalline Ni samples were then made by compaction and hot sintering. Experimental measurements on the mechanical response of bulk nanocrystalline Ni were performed under different strain rates and grain sizes. Comparison between experimental data and model predictions show that the method developed appears to be capable of describing the mechanical response of bulk nanocrystalline materials. The model applications to nanocrystalline Mg and Cu have shown that it can reflect the asymmetric defect development between tension and compression under quasi-static conditions; this results in its good capacity to describe the dynamic strain rate sensitivity and strain hardening behavior over a relatively large strain range under both compression and tension conditions.     

AFM characterization of the evolution of surface deformation during fatigue in polycrystalline copper

Atomic force microscopy (AFM) is a relatively new tool that readily provides high resolution digitized images of surface features. AFM is used here to study the development of slip bands and protrusions in strain controlled fatigue tests on polycrystalline copper at 0.161 and 0.255% strain amplitudes. The average slip band heights at failure for both strain amplitudes conditions are comparable, implying that the growth of slip bands saturates at a specific height. A parameter, γ_irrev, is defined that is a measure of the local slip irreversibility at the surface and is applicable to any type of surface deformation feature, independently of the size of the fields of view. Thus, estimates of surface deformation developed in regions where fatigue crack nucleation is likely to occur can be obtained, from which a fatigue crack nucleation criterion is defined.     

A thermodynamics model for solder profile evolution

A thermodynamics model for describing the solder profile evolution and the triple point line motion driven by surface tensions and gravity field is proposed. Attention is focused on the kinetic process as the solder profile evolves toward its equilibrium shape in its molten state. Based upon non-equilibrium thermodynamics, a theoretical model is proposed by introducing the kinetic laws that characterize the motions of the solder surface and the triple point line. The presented model leads to a nonlinear dynamic system, which describes the entire time-dependent process of the solder wetting. It can be applied not only to predict the equilibrium shape of the solder, but also the shape at any time during the spreading process. A computer numerical simulation and examples for the solution of the dynamic system are described.     

A size-dependent crystal plasticity finite-element model for creep and load shedding in polycrystalline titanium alloys

A rate-dependent anisotropic elastic-crystal plasticity based finite-element (FE) model with size-dependent yield strength is developed for polycrystalline Ti-6242. The initial slip system deformation resistances in the crystal plasticity relations are expressed as Hall–Petch type relations, where the grain size, lath size and colony size are chosen as characteristic lengths depending on the nature of slip. The FE model incorporates accurate phase volume fractions and orientation distributions that are statistically equivalent to those observed in orientation imaging microscopy (OIM) maps of the microstructure. The model is validated with experimental results on constant strain rate and creep tests. A relationship between the macroscopic flow stress and grain size and lath size for two-phase Ti-6242 is proposed. The effect of grain morphology on creep-induced load shedding and localization is discussed.     

A phase field model for electromigration-induced surface evolution

In this study, a phase field model is presented to study the effects of electromigration on the surface evolution of single crystal Al metal interconnects. Two dimensional computer simulations are performed for the surface evolution of metal interconnects due to electromigration in various conditions, such as anisotropy in diffusivity, different initial void sizes, and different crystallographic directions compared to the direction of the ambient electric field. From the results of computer simulations, it may be seen that the types of anisotropy and the relative direction of the diffusivity are the decisive factors in motion and shape change. As the symmetry of anisotropy in diffusivity decreases, the void evolves into a more unstable shape. Moreover, the voids of the system with two-fold diffusion symmetry are most likely to evolve into slits when the crystallographic direction is toward a specific orientation compared to the direction of the ambient electric field. This article is based on a presentation made in the 2002 Korea-US symposium on the “Phase Transformations of Nano-Materials,” organized as a special program of the 2002 Annual Meeting of the Korean Institute of Metals and Materials, held at Yonsei University, Seoul, Korea on October 25–26, 2002.     

An integrated computational support for design of system architecture and service

System architecting of mechatronic systems is to develop system decomposition and to define specifications of subsystems and interfaces. This study integrates a CAD for system architecting with a service CAD and life cycle simulation to model and reason about systems architecture and maintenance-related services as well as monetary flows between the manufacturer and user of systems in a life cycle. Such an integrated computational support promotes engineering design of life cycles and services in practice and useful to leverage mechatronics systems as service channels of product-service systems.     

AA6061铝合金超声波金属焊接计算分析模型

实现AA6061铝合金超声波金属焊接有效连接的关键机制是表面效应和体积效应。表面效应是焊接接头界面的摩擦;而体积效应存在于整个焊接过程中,影响了金属工件成形应力和形变。基于超声波焊接的关键机制建立了AA6061铝合金的材料模型和界面接触摩擦模型,依托ABAQUS有限元软件进行了铝合金超声波焊接的热-机耦合数值模拟分析。结果表明:与超声焊极相接触的铝合金界面产生最高温度和剧烈塑性变形,但低于母材熔点温度值。当焊极压力175 MPa,振幅8.4μm,加载时间60 ms时,界面最高温升至357.5℃,随着超声焊极压力持续增加铝合金表面发生粘焊。     

Phase-field simulation of polarization switching and domain evolution in ferroelectric polycrystals

A phase-field model is developed for predicting the polarization switching and domain structure evolution under an applied electric field in ferroelectric polycrystals. The model takes into account realistic grain structures as well as various energetic contributions, including elastic energy, electrostatic energy, and domain wall energy. A hysteresis loop – average polarization as a function of applied electric field – is computed, and the detailed domain evolution process during switching is analyzed. In particular, the role of grain boundaries in the nucleation and growth of new domains is studied. It is shown that switching takes place through the nucleation of 90° domains at grain boundaries and subsequent growth into the grain interiors instead of direct 180° domain switching. A correlation between the domain structures in neighboring grains was observed, and polarization switching in one grain was found to affect the switching in neighboring grains.     

A model for evolution of shape changing precipitates in multicomponent systems

Recently the authors introduced a concept of shape factors to extend an already established model for the growth and coarsening kinetics of spherical precipitates in multicomponent multiphase environments to needle- and disc-shaped geometries. The geometry of the precipitates is kept in the original version of the concept to be self-similar with a given fixed aspect ratio. In the present treatment, the aspect ratios of individual precipitates are treated as independent evolving parameters. The evolution equations of each precipitate, described by its effective radius, mean chemical composition and the aspect ratio, are derived by application of the thermodynamic extremal principle. The driving force for the evolution of the aspect ratio of the precipitate stems from the anisotropic misfit strain of the precipitate and from the orientation dependence of the interface energy. The model is used for the simulation of the precipitation of Ti₃AlN and Ti₂AlN in Ti–Al–0.5 at.% N matrix.     

注射模改模知识应用本体的建模方法

分析了注射模改模知识的特点和本体表示知识的方法,提出了构建应用本体的新模式,重点讨论了构造应用本体的克隆、合并、遗传,新建等4种建模方法,利用Protege软件建立了注射模改模知识应用本体,并转换为Web OWL语言。实践证明该方法能够较好地表示注射模改模知识。     

A 2-D mesoscopic model for the evaluation of creep damage induced by void growth in polycrystalline metals

At elevated temperature,creep failures of polycrystalline metals after long-term constant loading are often caused by the nucleation,growth and coalescence of creep voids on the grain boundaries.Void fraction is an important parameter for the evaluation of creep damage level of structural materials operating at high temperature.In this paper,a 2-D numerical simulation method was developed for analyzing the void fraction evolution during the process of creep,based on some hypothesis from experiments.The mode...     

A predictive model for microstructure evolution in metallic multilayers with immiscible constituents

Focussing on the thermal stability of layered structures, we developed a predictive model to study the microstructure evolution of metallic multilayers with different morphologies including aligned and classical staggered grain geometries. We found that the zig-zag microstructure experimentally observed in multilayers forms when grains in each upper layer have a relative shift less than half the in-plane grain size to the lower layer. During this formation process, the non-equilibrium triple junctions move, driven by the imbalance of tensions of interphase and grain boundaries, corresponding to an extension to classical grooving theory. Numerical simulations show that a finite mobility of the triple junction can effectively impede the development of grooves, suggesting that the classical t^1/4 dependence of groove depth with the assumption of an infinite triple junction mobility might be questionable at low temperatures to predict the time to pinch-off. Further, a map for the stability of layered structure in Cu/Nb system is developed in terms of the aspect ratio of grain dimensions and the ratio of the distance between two nearest triple junctions to the in-plane grain size. A criterion for this stability is also proposed for multilayers with similar grain boundary energies based on simplified geometrical consideration. Both the map and the simple criterion are in good agreement with the experiments for Cu/Nb multilayers.     

Constitutive model for casting magnesium alloy involving spherical void evolution

Casting magnesium alloys are highly heterogeneous materials inevitably containing numerous voids.These voids will evolve during material deformation and markedly affect material behaviors,so it is important to investigate the equation of the void evolution and the constitutive relation involving the void evolution.By assuming the voids in casting magnesium alloys were spherical,the growth equation of the voids was obtained from the incompressibility and continuity conditions of material matrix. Through c...